This invention relates to a semiconductor device operating as a phase canceller. In particular, the invention is related to a phase canceller for use in a carrier recovery network (CRN) of a time division multiple access transmission system (TDMA).
In such systems, the burst-to-burst frequency change may be in the order of .+-.5 k Hz. In the carrier recovery network, even if the automatic frequency control of the CRN cannot track these frequency variations, a 64-MHz band pass filter (BPF) will be responsive to those frequency variations together with phase shifts associated with them. In the case of utilizing high-Q filters, the phase shifts associated with such frequency variations may be as high as several degrees. Accordingly, in TDMA operation the bit error rate (BER) can be in the order of 1 dB. Accordingly, in such systems high loop gains are necessary to keep the phase error within acceptable limits.
In order to compensate for phase variations a phase-cancelling network can be inserted into the CRN loop. Such a loop is shown in FIG. 1. FIG. 1 shows a 70 MHz IF input signal delivered to a multiplier 12 with the input signals multiplied by 4 to generate a 280 MHz output signal. That signal is fed to a junction point 14 receiving the 280 MHz signal and also the output from a 54 MHz voltage controlled oscillator (VCXO 16) whose output is delivered to a multiplier section 18 thereby supplying a second input signal of 216 MHz to the junction point 14. The two signals are subtracted to derive a 64 MHz signal delivered to band pass filter 20.
The carrier recovery network shown in FIG. 1 then utilizes a threshold detector 22 as a portion of an automatic frequency control loop (AFC) together with phase detector 24 and amplifier 26. The phase detector 24 receives two input signals, one the output of summer 14 and the second the output of the threshold detector 22 to produce a control signal for the voltage controlled oscillator 16 to control frequency variation and hold the system at 64 MHz.
As indicated, the phase canceller of this invention is inserted into the CRN to compensate for phase variations and receives a control signal which is the output of the phase detector 24. The phase canceller 28 will be described in greater detail in the description of the preferred embodiment of this invention which follows. Its output is supplied to a divide by 4 divider 30 thereby producing a 16 MHz carrier signal. That output is added to the 54 MHz signal from oscillator 16 to produce, that is, recover the 70 MHz RC carrier signal.
Given this background of use of the phase canceller in accordance with the present invention, in general, the phase canceller can be defined as a constant resistance network operating as a linear phase shifting element. It can be defined by the transfer function G(s)=P.sub.1 (s)/P*.sub.2 (s), where, P.sub.1 (s) is the complex conjugate of P.sub.2 (s). That is, EQU .vertline.P.sub.1 (s).vertline.=.vertline.P.sub.2 (s).vertline.
arg *P.sub.1 (s)=-arg P.sub.2 (s)
It follows then that, EQU .vertline.G(s).vertline.=1 for all of .omega. and
.OMEGA.(.omega.)=arg G(s)
The network approximation of such a constant resistance network lies in finding a sensor element within the network that will change the parameters according to the phase error without disturbing the constant resistance structure. Moreover, constant resistance networks require a large number of circuit elements and usually are symmetrical structures which impose further difficulties in terms of substantive realization.
A linear shifting element can be approximated by a semiconductor device having the characteristics of a constant resistance network. Any semiconductor amplifying element, bipolar or field effect structure, acts as a delay element. For example, in conventional bipolar transistor elements minority carriers emitted from the emitter must cross the base region before they are collected by the collector. In the case of a FET, the same analogy is true since a finite time is needed for the carriers to travel from the source to the drain of such a field effect transistor. The velocity of the carriers and the distance between the emitter and the collector determine the time of travel. The distance between the emitter and collector can be selectively varied, that is increased or decreased, to satisfy the required phase shift for a given voltage. Increased distance results in increased attenuation. Attenuation however can be compensated by appropriate amplification. Similarly, for a given distance the electric field between the emitter and collector can be varied to alter the travel time.
Within the prior art a variety of semiconductor devices are known however, one operating as a phase shifting device in accordance with the criteria set forth above is not known. For example, U.S. Pat. No. 3,714,473 shows a charge carrier beam deflection device, that is a semiconductor device capable of controllably deflecting the charge carrier beam in a semiconductor material by applying an electric or magnetic field to the semiconductor material. FIGS. 1 and 2 of the '473 patent show such a semiconductor device utilizing plainer geometry PIN semiconductor structure with material zones 12 and 13 and deflector electrodes 15.1-15.4. A voltage source is shown as element 18 and a signal source as element 17.5. In operation, the signal source 17.5 provides an input signal at electrode 13.1 contacting zone 13. The presence or absence of the charge carrier beam in a semiconductor material is detected at the detector electrodes 15.1-15.4 depending on the voltage applied by the source 18 across the metal electrode 14. Metal electrode 14 is disposed in the P zone 12 and the contact 12.1 is located in the N+zone 13. Accordingly, in this semiconductor device a charge carrier beam in the semiconductor material is controlled by an applied voltage at ohmic contacts in the material. The patent however does not perceive of phase shifting a signal where phase change can be varied by changing the electric field applied between emitter and collector by varying the voltage applied across the ohmic contacts.
U.S. Pat. No. 3,810,049 relates to an integrated attenuation element having a semiconductor body 1 with zones 4, 5, and 6 of a P-type material in a zone 15 of an N-type material embedded in a semiconductor body 1. In operation, a high frequency input signal is provided at a contact electrode 7 in zone 4 and the output of this signal is detected at contact electrode 8 in zone 5. A voltage applied at terminal 47, connected to contact electrode 9 in zone 6 is used to control the attenuation of the high frequency signal transferred from input zone 4 to output zone 5. The patent, describes in column 4 lines 28-66 the characteristics of this semiconductor device wherein, the voltage applied at terminal 47 is capable of controlling the attenuation of the high frequency signal passing through the element. Accordingly, the -049 patent shows a device utilizing a control voltage to vary the characteristics of a high frequency signal passing through a semiconductor material. The patent however does not teach the concept of phase shifting the signal.
A number of other patents have been studied but are deemed to be structurally dissimilar from the present invention and not related to the function of linear phase shifting of a signal. Typical, are U.S. Pat. Nos. 4,032,916; 4,132,966; 3,313,952; 3,693,056; and 3,404,327, which all teach semiconductors wherein a voltage is applied to the semiconductor device to create an electric field within the device and thereby control the charge carrier flow to provide a variety of capabilities. Also, U.S. Pat. Nos. 3,649,847; 3,590,285; 3,808,517; and 4,122,364 teach generalized circuits for voltage control phase shifting of an electric signal. These patents however do not show a device related to a semiconductor phase shifter.